1. Field of Invention
The present invention relates to techniques for measuring properties of water using one or more sensors and/or samplers moving through the water guided by a wire.
2. Description of the Prior Art
The circulation of water bodies, such as the oceans, lakes and rivers, is driven by variation in density and boundary stresses. The water density in turn is largely determined by its temperature, salinity and other chemical attributes. A number of sensors are used to measure these attributes, among them thermometers and conductivity cells. A pressure sensor is also commonly used to determine the depth of the observations. A suite of such sensors are commonly mounted on a frame and called a profiler. This profiler apparatus, usually together with another apparatus mounted on the same frame for taking several water samples at various depths, is typically lowered from a ship or boat at the water surface to the floor of the body of water--a so-called hydrographic cast. The water samples are analyzed on shipboard or stored for subsequent shoreside chemical analyses. Present combined profiling and sampling systems are capable of capturing up to thirty-six 10-liter samples of water. Water depths can range from a few meters to nearly ten thousand meters.
Sensors generally have a finite response time, meaning that a certain interval of time is required before the sensor equilibrates with the water and a valid measurement can be made. Different sensors have different response times. Typically, it is an exponential convergence to the value. This difficulty is discussed in Horne et al, 1980, and Gregg et al, 1982, among others. In the case of a sensor moving through the water, this response time limits the lineal resolution possible. When the movement of the sensor through the water varies in speed, the sensor response time, and therefore the measurement precision, will vary along the traverse. This difficulty is discussed in Giles et al, 1986. Freely-falling, untethered profilers (for example, Hoyt and Bradley, 1988) obviate the problem of varying sensor speed, but do not generally have adequate ability to take water samples. For this reason, most of the hydrographic casts continue to be made with a sensor and sampler assembly on the end of a line or cable.
Limitations of the existing art include:
sensors moving through the water at varying speed giving rise to varying resolution in the measurement of the water properties, such as temperature, conductivity, chemical species, and depth. This variable speed also limits the ability to digitally filter the data to correct for sensor response mismatches. This is important when calculating derived parameters such as salinity, which is a function of temperature, conductivity and pressure. PA1 sensors connected to a wire and lowered from a boat at the surface which is heaving due to waves do not move through the water at a uniform speed because of the motion of the boat, thereby giving rise to varying resolution in the measurements due to the sampling interval not being uniform vertically. At the 25 Hz sampling rate and 1.5 m per second lowering rate commonly used, the vertical spacing of samples can vary from zero to ten centimeters, whereas a uniform sampling interval is desired. FIG. 2 is an example of a hydrographic cast in which the sampler reversed direction due to the heaving motion of the surface ship even though the winch was paying out wire at a constant speed. PA1 the flow of water past an object moving through it depends on its shape, size, inter-connected conduits and the speed. Depending on the speed at which the object is moving, the flow pattern often varies considerably, particularly with the complex shapes of hydrographic profilers. In some instants, the flow can actually reverse and the profiler will sense its wake rather than the surrounding water. In some cases, water may become entrained around the sensors instead of flushing by them. To reduce this problem, one manufacturer has a pump on their instrument to assure some movement of water past the sensor. Reduction of the variation in the speed of the moving sensor is a major step in obviating these changes and instabilities. PA1 the large size of the sampling profilers currently used has considerable drag. Some packages in current use have a cross-sectional area of as much as 1.2 sq. meters. The weight of the frame in air can be as much as 700 kg, and when the sample bottles are filled as much as 1000 kg. When the lowering rate varies, this gives rise to significant transient loads on the cable which have broken the electrical conductor or conductors in the cable, or parted the cable. The double-armored steel cables with one or three conductors commonly used are about 8 mm in diameter with a safe working load of 3000 kg. PA1 because of the relatively slow free-fall speed of the existing profilers, care must be taken not to pay out wire faster than the fall speed of the profiler, else the wire overrun the package and potentially foul on it. PA1 operations in heavy weather are difficult, if not impossible, because of the large dynamic loads on the profiler, wire, wire terminations, winch and fairlead.